Failure prediction of diamond film coated cutting

2022-08-11
  • Detail

Failure prediction of diamond film coated tools

diamond film coated tools are a high-tech product developed in recent years. Because diamond has the characteristics of high hardness, low friction coefficient and good thermal conductivity, the diamond film coated cutter has the advantages of high machining accuracy, long cutting life, small cutting force and high machining efficiency. Its cutting life is equivalent to that of polycrystalline diamond (PCD) tools currently used, which is 10 ~ 140 times that of cemented carbide tools. It is especially suitable for cutting non-ferrous metals, ceramics, fiber laminated composites, etc. PCD and cemented carbide tools often fail due to the serious wear of the flank and the increase of the arc radius of the tool tip, while diamond film coated tools usually fail due to the cracking, peeling and wear through of the diamond film coating. Because of this, the prediction of its failure is easy to realize, which is of great significance to reduce the waste and defective products when consumers finish drinking beverages in the cutting process and realize the automation of monitoring and inspection

comparative cutting test

the substrate of the diamond film coated tool is YG6 cemented carbide, the model is c116, and the arc radius of the tool tip is 0.3 ~ 0.8mm

in the process of CVD diamond film, the cobalt bonding phase in cemented carbide will promote the growth of graphite, so the surface of cemented carbide substrate must be pretreated to remove or reduce the cobalt content in the surface layer of the substrate. The general pretreatment process is as follows: firstly, the substrate surface is acid etched to remove cobalt, then ground with diamond powder and ultrasonically cleaned with acetone, then reduced to reduce carbon and deposited with ion beam assisted molybdenum transition layer and diamond-like carbon film

reduction and carbon reduction is to convert WC on the surface of the substrate into w or W2C. In the subsequent process of synthesizing diamond film, the carbon in the gas phase will convert w or W2C into WC to improve the bonding strength between the film and the substrate; The molybdenum transition layer can block the diffusion of cobalt, because after the cobalt on the surface of the substrate is removed by acid etching, in the process of CVD diamond, because the substrate temperature is as high as 700 ~ 900 ℃, cobalt will re diffuse to the surface, and even convert the synthesized diamond into graphite

diamond films were deposited on pretreated substrates by hot wire CVD with a film thickness of 5 ~ 6 μ m. The synthesized diamond grain is fine, uniform, dense and smooth, and the tool does not need to be polished again

turning the outer circle of automobile piston (f105mm) with PCD and cemented carbide tools at the same time × 110mm), the material of automobile piston is aluminum alloy containing 11% - 18% silicon. Cutting process parameters: cutting speed 300 ~ 700m/min; Feed rate 0.1mm/r; Cutting depth 0.2mm, dry cutting. Monitor the dynamic change of cutting force with an oscilloscope, and observe the chip condition at any time. After cutting, check the dimensional accuracy and surface quality of the workpiece and the tool failure. The results show that the cutting life of cemented carbide tools is the shortest, while the diamond film coated tools can cut continuously for 50 ~ 120min, and the total cutting length is 15 ~ 80km. Although its service life is not as long as that of PCD tools, the machined surface quality is better than that of PCD tools

failure form and prediction

the test shows that the main failure form of diamond film coated tools is film cracking and spalling (Fig. 1), followed by the wear through of the film. This is because:

1) in the process of CVD diamond film, the thermal expansion coefficients of matrix and film are different, and the lattice of matrix and film does not match, resulting in the generation of internal stress, which is improperly released when the coated tool is cutting

2) there is non diamond phase between the interface between the substrate and the film, especially when there is graphite phase, the adhesion and bonding strength between the film and the substrate are greatly reduced

3) with the increase of film thickness, the bonding strength between the film and the substrate decreases, so the thickness of diamond film is generally not greater than 20 μ m. Most are less than 10 μ m。 When the diamond film is too thin, the film will be worn through

In principle, in addition to the electron microscope of pvc/abs alloy 1 film peeling and cracking, the bonding strength between the diamond film and the substrate is the key factor restricting the development of diamond film coated tools. Although researchers at home and abroad have taken many technical measures and achieved certain results, this problem has not been fundamentally solved. At present, the peeling of diamond film is still the main form of its failure

because the piston excircle is a convex to elliptical surface, in the cutting test, when the piston excircle is processed with a diamond film coated tool, the cutting force changes periodically with time. It can be seen on the oscilloscope that the cutting force presents a regular waveform, and when the diamond film coated tool fails, the cutting force increases significantly, as shown in Figure 2

Fig. 2 waveform of cutting force before and after failure

when the diamond film coated tool fails, the film cracks and peels off or is worn through, and the cutting friction pair changes the diamond and workpiece material into cemented carbide YG6 and workpiece material, and the friction coefficient increases significantly, which intensifies the friction between the chip and the rake face, and bond under high pressure to form chip nodules. Measuring the dimensional accuracy and surface quality of the processed workpiece before and after the failure can be realized if the initial operation is relatively smooth. Before the failure, the processed dimensional accuracy and surface quality all meet the requirements. After the failure, the processed dimensional accuracy and surface quality will decline, and will not meet the processing requirements soon

pcd and cemented carbide tools will not change the cutting friction pair during cutting, and the change of cutting force mainly comes from the influence of tool flank wear. Some tests show that when the flank wear reaches 0.8 ~ 1.0 mm, the cutting force increases by about 30%. It is obviously difficult to predict tool failure based on this point. When the diamond film coated tool fails, as mentioned above, the cutting friction pair will change, and there will be chip buildup, and the cutting force will suddenly increase by 1 ~ 3 times, which creates favorable conditions for predicting tool failure. That is, the failure of diamond film coated tools can be judged by monitoring the generation of chip nodules and the change of cutting force in the cutting process. (end)

Copyright © 2011 JIN SHI